It is the Harvest Moon!

Here in New England, the long summer has abruptly receded, and a series of cool nights have signaled the arrival of fall. The maples, among the first to sense the changing seasons, have slowed their production of chlorophyll, and the first bright colored leaves have begun to peek through the canopy. This year, the changing of the seasons is coupled with a unique combination of astronomical events: Sunday’s full moon, the Harvest Moon, will additionally be both a supermoon and a “blood moon” lunar eclipse. These terms seem ripe for poetry on their own. But what do they really mean?

It is the Harvest Moon! On gilded vanes And roofs of villages, on woodland crests And their aerial neighborhoods of nests Deserted, on the curtained window-panes Of rooms where children sleep, on country lanes And harvest-fields, its mystic splendor rests!

Henry Wadsworth Longfellow’s poem, “The Harvest Moon,” depicts a still nighttime scene presided over by a Harvest Moon, in all its “mystic splendor.” The term “Harvest Moon” is applied to the full moon closest to the autumnal equinox. The Harvest Moon, which usually occurs in September, historically allowed farmers to continue their harvest into the night. Other, related, names for the September full moon include the Barley Moon and the Corn Moon.

The Harvest Moon is special for another reason. At the time of the Harvest Moon, the moon rises at about the same time for several nights, making it seem like the moon is full multiple nights in a row. This occurs because of the angle of the moon’s orbit relative to earth’s. Because the moon’s orbit is offset from earth’s orbit, the moon usually rises about 50 minutes later each successive night (that is, the time between sunset and when you see the moon increases by about 50 minutes from night to night). But around the time of the autumnal equinox, the moon’s orbital path makes a shallow angle with the horizon, so for a few days before and after the harvest moon, the moon rises only about 30 minutes later than it did the previous night. This happens around sunset, so what we see is a large, bright moon lingering around the eastern horizon just as it gets dark.

In 2015, our Harvest Moon is even more special than usual. This year, it is also a supermoon lunar eclipse, a phenomenon that hasn’t happened since 1982 and won’t happen again until 2033.

A supermoon is caused by the shape of the moon’s orbit. Because the orbit is elliptical, sometimes the moon is closer to the earth than other times. At its closest approach to earth, called perigee, the moon is about 31,000 miles closer to us than when it’s at its farthest point, called apogee. If a full moon coincides with perigee, we call it a supermoon because its proximity makes the moon look about 14 percent larger and 30 percent brighter than a normal moon, according to NASA.

Which brings us to the “blood moon” lunar eclipse. A lunar eclipse occurs when the earth lines up between the sun and the moon, blocking the sun’s light from falling on the moon. In the shadow of our planet, the moon appears reddish.

If the earth didn’t have an atmosphere, the moon might appear completely dark during an eclipse. Instead, sunlight bends around the earth and is filtered through the atmosphere, which removes blue light but allows red and orange light to reach the moon’s surface. What we see when a lunar eclipse is at its peak, is a blood-colored moon.

On Sunday, if the weather is clear, we should see a huge, bright moon that appears red for a few hours while it is eclipsed. Stand out in its light to celebrate, or mourn, the end of summer. The birds are leaving, the leaves are falling, and the chill wind wraps us as we reap whatever harvest this year has brought to us.

Gone are the birds that were our summer guests, With the last sheaves return the laboring wains! All things are symbols: the external shows Of Nature have their image in the mind, As flowers and fruits and falling of the leaves; The song-birds leave us at the summer’s close, Only the empty nests are left behind, And pipings of the quail among the sheaves.